feat: support github repo shorthand and scoped help

[travelhawk]

Summary

• accept bare OWNER/REPO GitHub shorthand across Git source and target locator flows and store it canonically as github:OWNER/REPO
• add interactive agentpm target add <locator> support that prompts for a target name with a repo-derived default
• make subcommand --help output command-specific and bump the workspace to 0.9.0 with changelog and docs updates

Validation

• pnpm test
• pnpm build
• pnpm smoke

Release Impact

• none
• patch
• minor
• major

Notes

• target add remains non-interactive-only when an explicit id is provided; locator-only usage now prompts in TTY mode and fails clearly otherwise.